This invention generally relates to power supplies and particularly to a power supply start-up circuit for use in connection with a switch mode power supply (SMPS).
It is common practice in computer installations to utilize SMPS power supplies because of their high efficiency, relatively low cost and light weight. A DC rail is generally established by conventional rectification of the AC voltage present on the main AC power lines. Additionally, the SMPS requires a start-up voltage in order to activate its pulse width modulator (PWM) circuit which controls the transistor switches coupled to the AC line. In prior art supplies, a start-up voltage is provided by a small 60 Hz transformer that delivers approximately 40 milliamperes of current at 15 volts. The transformer is undesirable for a number of reasons.
The transformer is the only device in the power supply that requires a pure sinewave voltage for proper operation. In many situations, it is highly desirable to have a supply that is capable of running from any voltage source, such as a standby power system. The transformer primary winding is often connected across one-half of a voltage doubler circuit to provide its primary winding with 110 volts AC when the power supply is connected to a 220 volts AC line. This can create a voltage imbalance across the main filter capacitors in the supply if the supply is shut down while AC line voltage is present at the supply input. This voltage imbalance can cause the voltage across one of the two capacitors in the voltage doubler circuit to exceed its voltage rating. The 60 Hz transformer also has a bulky iron core and presents packaging and circuit layout problems.
Because switch mode power supplies are not inherently isolated, the transformer is essential to provide isolation from the 60 Hz AC line. With the circuit of the invention, the 60 Hz transformer is replaced with a very small, light weight, ferrite core transformer that retains the necessary isolation. The transformer, which has a low voltage and current requirement, operates for a short period of time in an oscillator mode to develop a charge potential across a capacitor. When the capacitor has reached a sufficient charge potential, a discharge circuit is initiated to supply energy to start the pulse width modulator in the switch mode power supply. Means are also provided for disabling the transformer when the SMPS starts.